The present invention relates to a method and an apparatus for identification of different plastic materials by optical measurements, especially by spectroscopy analysis.
The plastics industry has experienced global growth in the past decades and this tendency will be continued in the future, as plastic products are used for a lot of products, being sold in increasing numbers. Especially casings for computers, laptops, screens, televisions, packaging materials, interior elements and devices in cars as well as external automotive parts, furniture, casings for electronic devices, etc. are manufactured from different plastic materials or even combinations thereof.
With the increasing manufacturing of plastic and plastic products, disposal and recycling of such plastic products have become a problem for the environment. Therefore, it is desired to recycle most of the plastic materials. For an effective recycling it is necessary that these plastic materials are identified and separated, as different materials require different and separated further treatments.
As sorting and identification techniques, different methods are known in the art, using e.g. properties such as density, electrical, magnetical, tripological or chemical separation. But, there are similar polymers, like co-polymers or polymer blends, as well as materials with different additives that cannot be separated by these methods.
Therefore, optical measurements, especially spectroscopic techniques have been developed. Different techniques are known in the art, as e.g. Near Infrared Reflection (NIR), Mid-Infrared Reflection (MIR), MIR Pyrolysis, MIR Acousto-Optic Tunable Filters (AOTF), RAMAN Scattering, or others. Among the above mentioned techniques, NIR, MIR and RAMAN are the techniques with the best reliability for identification of plastic materials, as used in modern products.
With the above mentioned or other spectroscopic measurements, samples are measured and sample spectra as well as reference spectra for specific plastic materials are provided. Normally, the raw data, achieved by the spectroscopic measurement, are further prepared and/or processed, e.g. by performing a Fourier Transformation, a base line correction, a vector normalization, etc., in order to make a further comparison of reference spectra and sample spectra easier and more reliable. These preparations of raw data can be performed e.g. by means of a computer together with respective computer programs.
After a sample spectrum has been measured and prepared or processed, it will be compared to reference data of all plastic materials of interest. Spectral distances between the sample spectrum and between each reference spectrum is determined, whereas the sample is supposed to be of the material with the reference spectrum that shows the minimum spectral distance, ideally the spectral distance is equal to 0.
Because the number of plastic materials of interest is possibly very large, a lot of comparing steps of the sample spectra with each reference spectra over the whole frequency range, e.g. in MIR between 400 and 4000 cmxe2x88x921, is necessary. Such a procedure is very time consuming, and the correct identification ratio is unsufficiently low, as the measured and achieved spectral distances do not clearly distinguish for some possible materials of interest.
It is therefore an object of the present invention to provide a method and an apparatus for identification of plastic materials of interest, wherein the procedure can be conducted in a less time consuming way and wherein more reliable results and therefore a higher correct identification rate can be achieved.
This object is solved by a method according to claim 1 and an apparatus according to claim 24. Claims 2 to 23 show preferred features of the inventive method of independent claim 1 and claims 25 to 26 show preferred embodiments of the apparatus according to claim 24.
According to the invention, at least one identification range having a high absolute deviation ratio D and/or a high smoothed deviation ratio Dxe2x80x2 between all pairs of possible plastic materials of interest is determined and a spectral distance, i.e. the added distance between two spectra to be compared over the relevant region, is only determined within said at least one identification ratio. A xe2x80x9chighxe2x80x9d ratio in this sense covers both high plus and minus values.
The absolute deviation ratio reflects a ratio between the absolute signal distances of the spectra of two materials to be compared and the consistency or noise and is therefore an indicator for the reliability of the measurement at the respective frequency for these materials. The identification frequency ranges are therefore those areas, where the distance between the absolute signals of the respective spectra to be compared is very high on the one hand and the noise is very low on the other hand, thereby leading to a high reliability. The noise may be measured by means of a standard deviation, when measuring a certain number of samples with the same molecular origin, but also any other value for the noise or consistency of the measurements can be used.
With the inventive method, only the frequency ranges, where spectral differences are present, will be investigated. Thereby a processing of areas, where the still remaining possible plastic materials of interest do not show remarkable or measurable differences, is omitted, thereby saving valuable measurement time.
Furthermore, and even more important, the reliability of a measurement results can be increased by comparing spectra only within limited ranges, as measurement noise will add up over a wide measurement range and may probably eliminate signal or spectra differences, making an identification impossible. Further, spectral distances that can be measured in a certain frequency range may add up to 0 with spectral distances in another frequency range, when measuring over the whole possible range, i.e. over more than the identification frequency range, as it is done according to the known methods of the state of art. The method according to the invention therefore avoids erroneous identification decisions.
The method according to the present invention has especially advantages, when polymers, containing additives, and similar plastic materials, having similar spectra over a wide frequency range, have to be identified.
It has been shown that with the present invention a reliability, i.e. a correct identification rate, of over 95 to 98% can be achieved within less than 2 seconds when identifying the standard main stream plastics.
Depending on the materials that have to be identified and separated, the reliability factor of the identification results is therefore up to 3 times better in comparison with a method according to the state of art.
When determining more than one identification frequency range and simply adding the spectral differences in all identification frequency ranges, it should be cross checked that there is no nullification or remarkable decreasing of the overall spectral difference (and therefore of the sum or integral of the deviation ratio over all identification frequency ranges) between all pairs of possible materials, as this might decrease measurement reliability. No problems will arise, when each identification frequency range is first considered separately and the overall spectral difference over all frequency ranges is determined by adding only absolute values |x|, i.e. positive values, of each identification frequency range, as spectral differences in each identification range can then only add up, when not taking into account different signs (plus/minus).
According to another aspect of the present invention, the method comprises at least two process or method levels, being conducted subsequently, wherein in each level the number of possible materials of interest is further limited. Within each level, the sample spectra, achieved by optical measurement, are only provided within a limited identification frequency range. This can either be achieved by measuring the samples only in these identification ranges in each level or by measuring the samples only once over a complete measurement range and further only providing the respective interesting frequency range for each level, what is the more preferred way.
The identification frequency ranges are determined in dependence of the group of plastic materials of interest in each level. The sample spectrum is compared with respective reference spectra only within these limited identification frequency ranges and the spectral distance is then determined, again only within these limited identification frequency ranges. Then at least two materials in the first level, at least one material in all levels starting with the second level and only one material in the last level for final identification having reference spectra with the smallest spectral distance is or are chosen. Thereby, a number of possible materials will be limited step by step until the final identification of the sample material in the last measurement or procedure level.
Such a procedure is especially useful, as some groups of plastic materials can easily be distinguished in the early levels, because they have clearly different spectra in certain identification frequency ranges. In the first levels, the number of possible materials is therefore very fast limited to a group of materials showing similar spectra.
Within the groups of materials with similar spectra, again, only the frequency ranges where spectral differences are present, will be investigated and measured, saving valuable measurement time and increasing reliability as explained above.
With a given group of materials of interest, it is preferable to separate specific sub-groups of materials and therefore provide a xe2x80x9cclusteringxe2x80x9d of sub-groups within one or more process levels, in order to provide better spectra differences for all materials in each sub-group and therefore have higher deviation ratios. Identification frequency ranges can then better be adapted for these limited number of materials in each sub-group, decreasing measurement time and increasing reliability.
Such a clustering can e.g. realize sub-groups with materials xe2x80x9ceasyxe2x80x9d to identify from each other and sub-groups containing materials xe2x80x9cdifficultxe2x80x9d to identify from each other. A criterion therefore can again be the deviation ratio between two materials. E.g. materials having a normalized deviation ratio over 2 or under xe2x88x922 being considered as easy to identify and therefore being in a first sub-group, and materials having a normalized deviation ratio between 1 and 2 or xe2x88x921 and xe2x88x922 respectively being difficult in identification and forming a second sub-group.
The inventive method therefore succeeds in the first levels in limiting the number of possible plastic materials of interest, thereby allowing especially in the further levels a specific determination and limitation of the identification frequency range, highly increasing the reliability and also decreasing the process time, in a comparison with the state of art.
It should be noticed at this point that it might also be possible that a material, showing a clearly distinctive spectrum in comparison to all other possible materials, can be identified directly after the first level.
The present invention has a special importance when identifying plastic materials that are used in modern products, as e.g. the group comprising ABS (Acrylnitril-Butadien-Styrol), HIPS (High Impact Polysterene), SAN (Styrene Acrylnitrile), PP (Polypropylene), PE (Polyethylene), PA (Polyamide), POM (Polyoxymethylene), PMMA (Polymethyl-Methacrylate), PC (Polycarbonate), PPO (Polyphenyloxide), combinations of PC and ABS, combinations of HIPS and PPO. These materials can be provided as essentially pure materials or they can comprise additives, especially hazardous additives like flame retardants, e.g. halogenated or phosphated flame retardants.
For a recycling process or for any other preparation of the materials, it is very important to know and to identify, which additives are comprised in a plastic material. As the spectra of the plastic materials comprising different additives do not show a clear spectral distance over the whole frequency range, the reliability or correct identification rate especially of these materials is much better with the inventive method in comparison with the known methods of the art.
It is especially preferred that the absolute deviation ratio D (X, Y, f), wherein X, Y are two of the possible plastic materials of interest, is determined by measuring a number N of different samples of the same molecular origin X, Y, numerically subtracting the N-weighted average of the measured signal S of the vibrational bands of sample Y from the N-weighted average of the measured signal S of the vibrational bands of sample X and normalizing by a term of the standard deviations or another value for the noise R of the sample X and Y measurements, wherein D is dependent of the measurement wavelength, the wavenumber or the frequency f.
The absolute deviation ratio is therefore determined according to the following formula:       D    ⁢          (              X        ,        Y        ,        f            )        =            [                        S          ⁢                      (                          X              ,              N              ,              f                        )                          -                  S          ⁢                      (                          Y              ,              N              ,              f                        )                              ]              [                        R          ⁢                      (                          X              ,              N              ,              f                        )                          +                  R          ⁢                      (                          Y              ,              N              ,              f                        )                              ]      
It is further possible to determine an integral deviation ratio Dxe2x80x2 (X, Y, f) wherein this integral deviation ratio is the average value of the absolute deviation ration D (X, Y, f) within a wavenumber or frequency range of fxe2x88x92xcex94f and f+xcex94f. xcex94f is normally smaller than 40 cmxe2x88x921, preferably smaller than 20 cmxe2x88x921, further preferably smaller than 10 cmxe2x88x921. Thereby a smoothing over 2 or 4 measurement points is achieved, depending on the measurement resolution.
According to a preferred embodiment of the present invention, the identification frequency ranges only comprise wavenumbers or frequencies, for which either the normalized value of the absolute deviation ratio D or the smoothed deviation ratio Dxe2x80x2 is higher than 1 or lower than xe2x88x921, for all pairs of possible materials of interest.
When using the absolute deviation ratio D for the determination of identification frequency ranges, possibly a lot of interrupted or small frequency ranges will occur, whereas when using the smoothed deviation ratio Dxe2x80x2, the respective graph of deviation ratio will be smoother, thereby leading to wider frequency ranges. Using the absolute deviation ratio will lead to still more accurate results, whereas using the smoothed deviation ratio will simplify the measurement or the controlling of the respective measurement devices.
According to the above described method, the identification frequency ranges as stated in claims 12 to 21 have been determined and proven to be useful with the respective group of possible plastic materials of interest.
As the materials of interest may vary depending on the application, e.g. depending on the company using the inventive method or the inventive apparatus and/or on the products to be recycled, it will be obvious to an artisan from the above explanation that combinations of process structures having different levels can be provided, in order to fit the inventive method to the desired application. Thereby different matrixes, i.e. different process levels with different identification frequency ranges can be combined to form a desired multi-level measurement matrix, being in accordance with the present invention.
One of the preferred measurement matrixes has in the first level an identification frequency range IFR1, measuring from 600 to 750 cmxe2x88x921, 850 to 1200 cmxe2x88x921, 1350 to 1500 cmxe2x88x921, 2750 to 3000 cmxe2x88x921 and an intermediate frequency range IFR2 measuring from 850 to 1100 cmxe2x88x921, 2150 to 2300 cmxe2x88x921 and 3000 to 3120 cmxe2x88x921. In the second level, an intermediate frequency range IFR3 measuring from 650 to 1800 cmxe2x88x921, 2150 to 2300 cmxe2x88x921 and 2750 to 3150 cmxe2x88x921, and an identification frequency range IFR4, measuring from 800 to 1440 cmxe2x88x921, 1470 to 1480 cmxe2x88x921, 1520 to 1570 cmxe2x88x921 and 1650 to 1750 cmxe2x88x921, will be used.
Depending on the results after the second level, a third level may be added, having a identification frequency range IFR5, from 650 to 1800 cmxe2x88x921 and 2750 to 3150 cmxe2x88x921, or an identification frequency range IFR6, from 850 to 1100 cmxe2x88x921, 1400 to 1800 cmxe2x88x921 and 3100 to 3300 cmxe2x88x921. This structure is especially useful and shows highly reliable identification rates when identifying ABS, HIPS, SAN with halogenated or phosphates flame retardants and PC+ ABS blends or HIPS+PPO blends.
This third level can be also directly integrated into the above described second level, forming only a 2-level measurement.
The invention also relates to an inventive apparatus, comprising a measurement device, measuring a sample and giving a sample spectrum, a first storage means, storing said sample spectrum and reference spectra for the possible materials, means providing sample spectrum and reference spectra only in at least one identification frequency range, a second storage means, storing and providing said sample and reference spectra only in said at least one identification frequency range, means, determining spectral distances between said sample spectrum and said reference spectra in the respective at least one identification frequency range, and means, associating the sample to at least one material having the reference spectrum or spectra with the smallest spectral distance to the sample spectrum.
The above mentioned first and second storage means can of course physically be the same means, not only separated means.
Such an apparatus is especially useful in operating a method as described above in a very efficient way and the advantages of the inventive method and the preferred procedures can be directly utilized by this apparatus.
In a preferred embodiment, the apparatus further comprises storage means storing multiple spectra of each of at least two materials and means determining an absolute deviation ratio D and/or an smoothed deviation ratio Dxe2x80x2 of groups of each two of the materials of interest.
It is advantageous that the apparatus further comprises means comparing said deviation ratios D and/or Dxe2x80x2 and determining at least one identification frequency range, for which the normalized value of the absolute deviation ratio D and/or of the smoothed deviation ratio Dxe2x80x2 is higher than 1 or lower than xe2x88x921 for all pairs of possible materials of interest.
With these features of the apparatus, an integral apparatus, being able to conduct all operations for achieving a high identification rate without additional external means, is provided, realizing a powerful tool for identification of plastic material, being difficult to identify with apparatuses according to the state of art.